The invention relates to a method for calibrating and operating a vacuum measuring cell arrangement, as well as to a method for operating a calibrated measuring cell arrangement.
At least three methods are known and feasible for the calibration of measuring cells. While within the metrology of a national standards institute, as for example the Physical-Technical Federal Institute (PTB, Germany), the methods with static and dynamic expansion are conventional, in the industrial manufacturing of measuring cells the approach of comparison with a transfer standard is preferably applied. The reasons are primarily the complexity of construction systems for the expansion methods, and particularly the time necessary to carry out a calibration using these methods. However the calibration method of the comparison with a transfer standard also requires taking into consideration some ancillary conditions, such as are stipulated in the standards DIN 28418 and DKD-R 6-2 or ISO/TS 3567:2005. The calibration of vacuum measuring cells is described for example in the literature citation Wutz-Adam-Walcher (Verlag Vieweg, ISBN 3-528-04884-0) in Chapter 11.8 Calibration of Vacuum Gauges.
It is known to measure pressures or pressure differences by pressurizing a thin diaphragm and measuring its deflection. A known and suitable method for measuring the deflection of such diaphragms comprises implementing the diaphragm arrangement as a variable electric capacitor, wherein, via an electronic measuring circuitry, the capacitance change is analyzed in known manner, which change correlates with the pressure change. The capacitor is formed by disposing the thin flexible diaphragm surface at a minimal spacing opposite a further surface and coating both opposing surfaces with an electrically conducting coating or developing them of an electrically conductive material. Upon pressure being applied to the diaphragm, the spacing between the two electrodes changes through the deflection leading to an analyzable capacitance change of the arrangement. Sensors of this type are produced of silicon in large piece numbers. The areal base body as well as also the diaphragm are herein often entirely comprised of silicon material. There are also designs with combined material composition, for example silicon with a glass substrate. The sensors can thereby be produced cost-effectively. As a rule, pressure sensors of this type are only applicable for higher pressure ranges in the range of approximately 10−1 mbar to a few bar. High resolution at lower pressures starting at approximately 10−1 mbar are no longer realizable utilizing silicon as the material. Sensors of this type are only conditionally suitable for typical vacuum applications. One of the reasons is that silicon reacts on its surface with the environment and in this way the sensitive sensor characteristic is disturbed. Water vapor contained in normal atmospheric air already leads to corresponding reactions on the surfaces. The problem is additionally exacerbated if the sensor is employed in chemically aggressive atmospheres, which is increasingly common in current reactive vacuum plasma processes.
One important application field in general are processes in the semiconductor industry. Here, semiconductors are produced utilizing, for example, the following techniques: chemical vapor deposition (CVD), physical vapor deposition (PVD), implanting and (dry) etching processes. Typical pressure ranges for processes in the semiconductor industry and pressure ranges of vacuum measuring cells typically operate in the range of 10−4 to 10 mbar. Typical process measuring cells for the applications are capacitive diaphragm measuring cells. In such processes, such as for example in vacuum etching methods, in particular especially aggressive media, such as fluorine, bromic acid and their compounds are employed. Due to such corrosion and resistance problems, the known silicon pressure sensors and diaphragm measuring cells with metallic diaphragms can only be employed to a limited extent.
For such applications there is increasing demand for being able to operate the diaphragm measuring cell at increased temperatures in order to be able to operate the measuring cell, for one, in a hot process environment and/or to avoid as much as feasible condensates in the measuring cell and to do this at high corrosion resistance.
There is expectation that the market demand for high-temperature diaphragm measuring cells will increase over the next years, for example due to the introduction of atomic layer deposition (ALD) in semiconductor production processes, which require pressure measurements at temperatures up to 300° C. or higher in certain applications. The apparatus structure for ALD processes is very similar to that of low pressure CVD (LPCVD) or CVD apparatus, which today are the most significant purchasers of measuring cells operated at increased temperatures.
A diaphragm measuring cell preferred for these applications is the capacitive diaphragm measuring cell (CDG). A capacitive diaphragm measuring cell, also referred to as capacitance diaphragm gauge (CDG), is based on the elastic deformation of a thin diaphragm, which is suspended over a solid, areal body and thus separates two volumes from one another. A pressure change in these volumes induces the diaphragm to move. The distance between the housing and the diaphragm changes. At high pressures the diaphragm is deflected more strongly than at low pressures. Metallic electrodes are disposed in the gap region on the diaphragm and on the base body which is located opposite the diaphragm. These two metal electrodes form a condenser capacitance. The capacitance change is consequently a measure of the pressure change. This measuring principle is independent of the type of gas.
It has therefore been proposed to produce measuring cells for vacuum pressure measurements of corrosion-resistant materials such as Al2O3. U.S. Pat. No. 6,591,687 B1 describes a capacitive vacuum measuring cell (CDG) which is substantially structured entirely of ceramic and thus is highly corrosion resistant. The content of this patent is herewith declared to be an integrated component of the present invention description. In order to measure, for example, very low pressures to 10−6 mbar with high accuracy, a very thin ceramic diaphragm of 25 μm to 100 μm thickness is preferably utilized, which is disposed substantially symmetrically in a ceramic housing. For the application at higher vacuum pressures up into the range of several 100 mbar, diaphragm thicknesses up to 950 μm are, for example, preferred. This diaphragm-based vacuum measuring cell is commercially highly successful and indicates a significant advance with respect to corrosion resistance.
A further preferred diaphragm measuring cell arrangement is based on the above described measuring cell of Al2O3 and utilizes a similar structure, wherein the degree of deflection of the diaphragm in this case takes place with the aid of optical means. In an optical diaphragm measuring cell, also referred to as optical diaphragm gauge (ODG), the pressure-dependent deflection of the diaphragm is measured in the sensor with the aid of an optical system, wherein the measured signal is conducted using fiber optics to the optical signal processing unit, which subsequently converts the optical signal into an electrical signal. The coupling-in of the light necessary for this purpose takes place directly onto the diaphragm via appropriately light-permeable regions on the housing of the sensor. From here the light is reflected back. The arrangement forms part of an interferometric Fabry-Perot system. In the associated interferometer through the signal analysis the degree of diaphragm deflection is measured, which is the measure of the obtaining vacuum pressure to be measured. The optical windows are advantageously produced of sapphire such that at least portions of the housing of the diaphragm vacuum measuring cell comprise sapphire. It is also advantageous if the diaphragm itself is comprised of sapphire. The optical signal can be conducted, for example, over large distances (even kilometers), with very low attenuation and without falsification through ambient disturbances, such as primarily electromagnetic interferences, vibrations and changes of ambient temperatures. Such a measuring cell can also be operated especially well as a heated measuring cell. A preferred disposition of an optical vacuum measuring cell has been described in the US application 2007 0089524 A1. The content of this patent application is herewith declared to be an integrated component of the present invention description.
A further improvement of the service life of such diaphragm measuring cells comprises that the connection regions between diaphragm and housing, as well as of the connection region for the connection fitting, and optionally the connection fitting itself, even when employed in aggressive process environments containing, for example, acids, halogens such as chlorine and fluorine, are covered and protected additionally with a thin corrosion-resistant layer. The deposition of such a protective layer, preferably of a metal oxide, is advantageously carried out with the aid of an ALD method, as is proposed in the patent application CH 01817/06. The content of this patent application is herewith declared to be an integrated component of the present invention description.
As already stated, in processes with aggressive gases, under especially high requirements made of measuring accuracy and long-term stability, heated measuring cells are preferably employed. Condensate depositions, for example, can thereby be decreased or avoided in regions within the measuring cell exposed to the process environment. Through the precise stabilization of the measuring cell temperature, instabilities through temperature effects can also be compensated. For this purpose correspondingly high complexities and costs are expended. For example, heating jackets, such as foil heating elements or heating tapes, are placed about the measuring cell which, in turn, are insulated in complex manner. The requisite electronic measuring circuitry, in turn, must be protected against these temperatures, for example by disposition at a spacing and through additional cooling measures, such as using ventilators and cooling bodies. Often additional heating elements, such as heating tapes, are utilized for heating the tubular inlets to the measuring cell. The temperatures are set to fixedly graduated values, such as for example 45° C., 100° C., 160° C. and 200° C., depending on the employment range for the processes to be measured. An arrangement especially suitable for heated diaphragm vacuum measuring cells with a heating system is described in the Swiss application CH 00985/07 by the same applicant. Herein a diaphragm vacuum measuring cell is disposed within a thermal container, which forms a heating configuration and thereby heats the measuring cell to the desired temperature, wherein the measuring cell connection for the vacuum pressure measurement is carried through the thermal container and in this region the thermal container is implemented as a thermal body in which a heat source is disposed. The thermal container is encompassed by an insulation jacket in order to insulate the heated thermal body against the environment and thereby to ensure the lowest feasible temperature gradients in the thermal container at low heat losses. Hereby a homogeneous temperature distribution on the measuring cell is made feasible at compact construction. The content of this patent application is herewith declared to be an integrated component of the present invention description.
Diaphragm measuring cells of aforementioned type supply a very small electric output signal which, accordingly, must be processed carefully. These measuring cells are also very sensitive, in particular to temperature changes. In the production, deviations occur with each measuring cell and especially so during operation at different increased temperature values. The more precisely the pressure measurements must be carried out, the more weight they carry and must therefore be taken into consideration correspondingly.
The measuring cells are therefore calibrated during their production and specially each in the operating points for which specified working application the measuring cell is intended. Calibration denotes the measurement of a test sample against a measurement standard or against a reference. Herein only the state is detected; the test sample is not, however, set to a standard state.
This setting is carried out in a separate step and is referred to as gauging. This gauging process means setting the test sample to a standard state. This action should subsequently preferably be followed by a further calibration. It is consequently important to differentiate between the two processes ‘calibration’ and ‘gauging’.
According to the above definition, the following steps are conventionally performed in a calibration means: measuring the test sample against a standard, storing the data, calculating the compensation values, gauging the test sample, calibrating the test sample with the simultaneous preparation of the calibration report.
Each measuring cell must in particular also be calibrated to the appropriate intended measuring cell temperature. The calibration process, as stated, comprises a comparison measurement of the measuring cell to be tested against a standard, thus against a reference measuring cell. Herein the state is detected and the deviations for the desired measuring range is recorded. The determined deviations can subsequently be utilized to correct or to gauge accordingly the measuring signals generated by the measuring cell.
Such measuring cells can thus always only be operated at this precisely defined temperature for which it has been gauged. For each value of a desired measuring cell temperature in each instance a measuring cell specifically calibrated to it must be employed by the operator at the vacuum process unit. Depending on the applied process and the requirements, if another working temperature of the measuring cell is required, another measuring cell must be employed in each case which has specifically been calibrated for this purpose. This increases the complexity considerably and the operating temperature of the measuring cell cannot be simply changed by the operator on site.